The desiderata for this type of buckle are; free movement of the strap in the released condition of the buckle without, however, risk of the loss of the strap or of the buckle falling apart and; in the closed condition of the buckle a positive and foolproof closure which cannot be accidentally knocked loose but which allows the unidirectional movement of the strap through the buckle.
The commononest type of such buckle is a form where two metal loops of slightly different dimensions are pivotally secured to the anchor point and the strap is reeved around the loops. It can be released for free movement by manually pulling the loops apart; it is always moveable in one direction through the loops but in the other direction the loops are pressed together and resist the movement of the strap. The disadvantages of this type of construction are its lack of positiveness and the ease with which the strap can become lost or folded.